devtools/postsubmit: find changes against the second latest snapshot.
devtools/postsubmit: find changes against the second latest snapshot.

The second latest snapshot is produced by the last "jiri update" run.

Change-Id: I06c2ce5182f1283445481693d856f88888b618cf

vanadium/go.devtools,vanadium/go.devtools,vanadium/go.devtools